1. Technical Field
The present disclosure relates to a current mode control type switching power supply device. In particular, it relates to a switching power supply device, which performs slope compensation in order to suppress subharmonic oscillation.
2. Description of Related Art
When a switching element is operated with an ON duty cycle (ON-time ratio) of 50% or more in a current mode control type switching power supply device, a phenomenon may occur in which a switching current fluctuates at a lower frequency than at a switching operation frequency (phenomenon where the ON duty cycle is not stable but swings). This phenomenon is referred to as subharmonic oscillation.
The subharmonic oscillation brings about disadvantages such as an increase of ripples, occurrence of noise, etc. Therefore, a switching power supply device in which slope compensation is performed to suppress subharmonic oscillation has been proposed, for example, in JP-A-2004-040856 and JP-A-2006-149065.
There are two types of slope compensation, that is, a first type in which a downslope signal is added to a signal in a feedback signal form (signal indicating an output voltage error) and a second type in which an upslope signal is added to a current detection signal (signal corresponding to a current flowing into a switching element).
A start point (start timing) of the slope compensation is generally set by use of a signal of an internal oscillator and applied to a region in which the switching ON duty cycle is more than 50%. The slope compensation is applied from the start point to suppress subharmonic oscillation so that stable operation can be performed.
Fluctuation in the region where the slope compensation starts affects the accuracy in making a determination as to whether an output current is an overcurrent or not (overcurrent determination accuracy). This is described as follows.
When a signal which has been subjected to slope compensation is used to determine the overcurrent, a reference value for making the determination is the sum of an actual current determination value (determination value for a detected current) and a slope compensation value at the time of the determination. To determine (ascertain) the slope compensation value in this case is as follows:
1) When the design of a switching power supply device is determined, the relationship between an ON-time ratio (ON duty cycle) and the heaviness of a load (output current value) is fixed.
2) The ON-time ratio for determining the overcurrent is determined.
3) The slope compensation value is determined in response to the ON-time ratio being determined. The slope compensation value is set when a reference value (overcurrent determination reference value) for determining the overcurrent is determined.
When the start timing of the slope compensation deviates, a difference between the slope compensation value determined in the paragraph 3) and the actual slope compensation value is generated. Accordingly, overcurrent determination accuracy decreases when using the overcurrent determination reference value.
A slope compensation circuit in the switching power supply device is described in JP-A-2004-040856. This slope compensation circuit is configured with an amplification circuit capable of offsetting a predetermined voltage. The terminal voltage of a capacitor, charged by a current from a constant current source, is input to the amplification circuit to obtain a slope compensation signal, which rises at a predetermined timing. The amplification circuit outputs a triangular wave signal as a slope compensation signal. The triangular wave signal increases in accord with a predetermined gradient from the instant the terminal voltage of the capacitor is coincident with the offset voltage. The slope compensation signal is added to a current detection signal indicating the magnitude of a current flowing into a switching element and is used for PWM control of the switching element.
In the slope compensation circuit of JP-A-2004-040856, there is a concern that a start timing of slope compensation may vary due to: a variation in the current value of the constant current source charging the capacitor, a capacitance value variation in the capacitor, a voltage value variation in a reference voltage source determining the offset voltage of the amplification circuit, etc. The variation in the start timing of slope compensation causes the overcurrent determination accuracy to decrease.
On the other hand, JP-A-2006-149065 describes a circuit that charges/discharges a plurality of capacitors, and determines a rising timing of a slope compensation signal (i.e. a start timing of slope compensation) based on a comparison among charging voltages of the respective capacitors.
Also in this circuit, there is a concern that the start timing of slope compensation may vary due to: a capacitance value variation in each capacitor, a current value variation in a constant current source charging the respective capacitors, an offset variation of a comparator comparing the charging voltages of the capacitors, etc. The variation in the start timing of slope compensation causes the overcurrent determination accuracy to decrease.